Objectives: Primary pulmonary hypertension (PPH) is a pulmonary vasculopathy resulting in exercise intolerance, usually due to dyspnea. We hypothesized that ventilation is increased during exercise in PPH relative to normal because the ventilated lung is underperfused, cardiac output increase is restricted, and arterial hypoxemia may develop. Our aim was to determine the size of the reduction in end-tidal Pco2 (Petco2) as a reflection of the abnormality in ventilatory efficiency and ventilatory drive in PPH patients.
Methods: We performed cardiopulmonary exercise testing (CPET) in 52 PPH patients. All had hemodynamic measurements to confirm the diagnosis of PPH. A subgroup of 29 patients who underwent right-heart catheterization within 50 days of CPET were studied to compare their CPET responses to resting hemodynamics. Nine healthy volunteers matched for age and gender served as CPET control subjects.
Results: In PPH patients, the percentage of predicted peak oxygen uptake (V̇o2) correlated significantly with mean pulmonary artery pressure (mPAP) [r = − 0.59, p = 0.0007, n = 29]. Petco2 values at rest, anaerobic threshold (AT), and peak V̇o2 were proportionately reduced as percentage of predicted peak V̇o2 decreased (r = 0.66 to 0.72, p < 0.0001, n = 52). Petco2 values at rest, AT, and peak V̇o2 were also reduced as mPAP increased (r = − 0.51 to − 0.53, p < 0.005, n = 29). In contrast to normal subjects in whom Petco2 increased from rest to AT, Petco2 decreased in PPH patients, except for two patients with mild PPH in whom there was no change. Also, Petco2 increased rather than decreased further at the start of recovery, in contrast to normal. Although usually normal at rest, oxyhemoglobin saturation decreased during exercise in most PPH patients.
Conclusions: In patients with PPH, Petco2 at rest and exercise is significantly reduced in proportion to physiologic disease severity. The range of values is unusually low. Furthermore, the directional changes of Petco2 during exercise and early recovery are in the opposite direction of normal.